Group isomorphism of intersections

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manpreet Tuteehub forum best answer Best Answer 3 years ago

In the syllabus for my course on group theory, there is the following theorem which is proven: "If HZnH⊂Zn is a subgroup, then HZkH≅Zk for 0kn0≤k≤n".

I understand most of the proof, but there's a simple line that is intuitive, yet doesn't mathematically make sense to me.

In the proof, which is done by induction, the following homomorphism is defined

π:Zn+1Z:π(m1,...,mn+1)=mn+1.π:Zn+1→Z:π(m1,...,mn+1)=mn+1.

The kernel of the map is considered, which are all vectors of the form (m1,...,mn,0)(m1,...,mn,0).

 

Then the syllabus reads "Since HZn+1H⊂Zn+1 is a subgroup, also H Ker

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manpreet 3 years ago

You actually already said the answer. Ker(π)Ker(π) is isomorphic to ZnZn and HKer(π)H∩Ker(π) is a subgroup of Ker(π)Ker(π), and so we can consider it as a subgrouop of ZnZn.


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